/*

-Procedure subslr_c ( Sub-solar point )

-Abstract
 
   Compute the rectangular coordinates of the sub-solar point on 
   a target body at a specified epoch, optionally corrected for 
   light time and stellar aberration. 
 
   The surface of the target body may be represented by a triaxial 
   ellipsoid or by topographic data provided by DSK files. 
 
   This routine supersedes subsol_c. 
 
-Disclaimer
 
   THIS SOFTWARE AND ANY RELATED MATERIALS WERE CREATED BY THE 
   CALIFORNIA INSTITUTE OF TECHNOLOGY (CALTECH) UNDER A U.S. 
   GOVERNMENT CONTRACT WITH THE NATIONAL AERONAUTICS AND SPACE 
   ADMINISTRATION (NASA). THE SOFTWARE IS TECHNOLOGY AND SOFTWARE 
   PUBLICLY AVAILABLE UNDER U.S. EXPORT LAWS AND IS PROVIDED "AS-IS" 
   TO THE RECIPIENT WITHOUT WARRANTY OF ANY KIND, INCLUDING ANY 
   WARRANTIES OF PERFORMANCE OR MERCHANTABILITY OR FITNESS FOR A 
   PARTICULAR USE OR PURPOSE (AS SET FORTH IN UNITED STATES UCC 
   SECTIONS 2312-2313) OR FOR ANY PURPOSE WHATSOEVER, FOR THE 
   SOFTWARE AND RELATED MATERIALS, HOWEVER USED. 
 
   IN NO EVENT SHALL CALTECH, ITS JET PROPULSION LABORATORY, OR NASA 
   BE LIABLE FOR ANY DAMAGES AND/OR COSTS, INCLUDING, BUT NOT 
   LIMITED TO, INCIDENTAL OR CONSEQUENTIAL DAMAGES OF ANY KIND, 
   INCLUDING ECONOMIC DAMAGE OR INJURY TO PROPERTY AND LOST PROFITS, 
   REGARDLESS OF WHETHER CALTECH, JPL, OR NASA BE ADVISED, HAVE 
   REASON TO KNOW, OR, IN FACT, SHALL KNOW OF THE POSSIBILITY. 
 
   RECIPIENT BEARS ALL RISK RELATING TO QUALITY AND PERFORMANCE OF 
   THE SOFTWARE AND ANY RELATED MATERIALS, AND AGREES TO INDEMNIFY 
   CALTECH AND NASA FOR ALL THIRD-PARTY CLAIMS RESULTING FROM THE 
   ACTIONS OF RECIPIENT IN THE USE OF THE SOFTWARE. 
 
-Required_Reading
 
   DSK 
   FRAMES 
   NAIF_IDS 
   PCK 
   SPK 
   TIME 
 
-Keywords
 
   GEOMETRY 
 
*/

   #include "SpiceUsr.h"
   #include "SpiceZfc.h"
   #include "SpiceZmc.h"
   #undef subslr_c

 
   void subslr_c ( ConstSpiceChar       * method,
                   ConstSpiceChar       * target,
                   SpiceDouble            et,
                   ConstSpiceChar       * fixref,
                   ConstSpiceChar       * abcorr,
                   ConstSpiceChar       * obsrvr,
                   SpiceDouble            spoint [3],
                   SpiceDouble          * trgepc,
                   SpiceDouble            srfvec [3] )
/*

-Brief_I/O
 
   Variable  I/O  Description 
   --------  ---  -------------------------------------------------- 
   method     I   Computation method. 
   target     I   Name of target body. 
   et         I   Epoch in ephemeris seconds past J2000 TDB. 
   fixref     I   Body-fixed, body-centered target body frame. 
   abcorr     I   Aberration correction. 
   obsrvr     I   Name of observing body. 
   spoint     O   Sub-solar point on the target body. 
   trgepc     O   Sub-solar point epoch. 
   srfvec     O   Vector from observer to sub-solar point. 
 
-Detailed_Input
 
   method   is a short string providing parameters defining 
            the computation method to be used. In the syntax 
            descriptions below, items delimited by brackets 
            are optional. 
 
            `method' may be assigned the following values: 
 
               "NEAR POINT/ELLIPSOID" 
 
                  The sub-solar point computation uses a triaxial 
                  ellipsoid to model the surface of the target body. 
                  The sub-solar point is defined as the nearest 
                  point on the target relative to the sun. 
 
                  The word "NADIR" may be substituted for the phrase 
                  "NEAR POINT" in the string above.  
 
                  For backwards compatibility, the older syntax 
 
                     "Near point: ellipsoid" 
 
                  is accepted as well. 
 
 
               "INTERCEPT/ELLIPSOID" 
 
                  The sub-solar point computation uses a triaxial 
                  ellipsoid to model the surface of the target body. 
                  The sub-solar point is defined as the target 
                  surface intercept of the line containing the sun 
                  and the target's center. 
 
                  For backwards compatibility, the older syntax 
 
                     "Intercept: ellipsoid" 
 
                  is accepted as well. 
 
 
               "NADIR/DSK/UNPRIORITIZED[/SURFACES = <surface list>]" 
 
                  The sub-solar point computation uses DSK data to 
                  model the surface of the target body. The 
                  sub-solar point is defined as the intercept, on 
                  the surface represented by the DSK data, of the 
                  line containing the sun and the nearest point on 
                  the target's reference ellipsoid. If multiple such 
                  intercepts exist, the one closest to the sun is 
                  selected. 
 
                  Note that this definition of the sub-solar point 
                  is not equivalent to the "nearest point on the 
                  surface to the sun."  The phrase "NEAR POINT" may 
                  NOT be substituted for "NADIR" in the string 
                  above. 
 
                  The surface list specification is optional. The 
                  syntax of the list is 
 
                     <surface 1> [, <surface 2>...] 
 
                  If present, it indicates that data only for the 
                  listed surfaces are to be used; however, data 
                  need not be available for all surfaces in the 
                  list. If absent, loaded DSK data for any surface 
                  associated with the target body are used. 
 
                  The surface list may contain surface names or 
                  surface ID codes. Names containing blanks must 
                  be delimited by escaped double quotes, for example 
 
                     "SURFACES = \"Mars MEGDR 128 PIXEL/DEG\""
 
                  If multiple surfaces are specified, their names 
                  or IDs must be separated by commas. 
 
                  See the Particulars section below for details 
                  concerning use of DSK data. 

 
               "INTERCEPT/DSK/UNPRIORITIZED[/SURFACES = <surface list>]"  
 
                  The sub-solar point computation uses DSK data to 
                  model the surface of the target body. The 
                  sub-solar point is defined as the target surface 
                  intercept of the line containing the sun and the 
                  target's center. 
 
                  If multiple such intercepts exist, the one closest 
                  to the sun is selected. 
 
                  The surface list specification is optional. The 
                  syntax of the list is identical to that for the 
                  NADIR option described above. 
 
 
               Neither case nor white space are significant in 
               `method', except within double-quoted strings. For 
               example, the string " eLLipsoid/nearpoint " is valid. 
 
               Within double-quoted strings, blank characters are 
               significant, but multiple consecutive blanks are 
               considered equivalent to a single blank. Case is  
               not significant. So 
 
                  "Mars MEGDR 128 PIXEL/DEG" 
 
               is equivalent to  
 
                  " mars megdr  128  pixel/deg " 
 
               but not to 
 
                  "MARS MEGDR128PIXEL/DEG" 
 
                
   target      is the name of the target body. The target body is  
               an ephemeris object (its trajectory is given by 
               SPK data), and is an extended object. 
 
               The string `target' is case-insensitive, and leading 
               and trailing blanks in `target' are not significant. 
               Optionally, you may supply a string containing the 
               integer ID code for the object. For example both 
               "MOON" and "301" are legitimate strings that indicate 
               the Moon is the target body. 
 
               When the target body's surface is represented by a 
               tri-axial ellipsoid, this routine assumes that a 
               kernel variable representing the ellipsoid's radii is 
               present in the kernel pool. Normally the kernel 
               variable would be defined by loading a PCK file. 
 
 
   et          is the epoch of participation of the observer, 
               expressed as ephemeris seconds past J2000 TDB: `et' is 
               the epoch at which the observer's state is computed. 
 
               When aberration corrections are not used, `et' is also 
               the epoch at which the position and orientation of 
               the target body and the position of the Sun are 
               computed. 
 
               When aberration corrections are used, `et' is the epoch 
               at which the observer's state relative to the solar 
               system barycenter is computed; in this case the 
               position and orientation of the target body are 
               computed at et-lt, where `lt' is the one-way light time 
               between the sub-solar point and the observer. See the 
               description of `abcorr' below for details. 
 
 
   fixref      is the name of a body-fixed reference frame centered 
               on the target body. `fixref' may be any such frame 
               supported by the SPICE system, including built-in 
               frames (documented in the Frames Required Reading) 
               and frames defined by a loaded frame kernel (FK). The 
               string `fixref' is case-insensitive, and leading and 
               trailing blanks in `fixref' are not significant. 
 
               The output sub-solar point `spoint' and the 
               observer-to-sub-solar point vector `srfvec' will be 
               expressed relative to this reference frame. 
 
                
   abcorr      indicates the aberration correction to be applied 
               when computing the target position and orientation 
               and the position of the Sun. 
 
               For remote sensing applications, where the apparent 
               sub-solar point seen by the observer is desired, 
               normally either of the corrections 
             
                  "LT+S"  
                  "CN+S" 
    
               should be used. These and the other supported options 
               are described below. `abcorr' may be any of the  
               following: 
 
                  "NONE"     Apply no correction. Return the 
                             geometric sub-solar point on the target 
                             body. 
 
               Let `lt' represent the one-way light time between the 
               observer and the sub-solar point (note: NOT between 
               the observer and the target body's center). The 
               following values of `abcorr' apply to the "reception" 
               case in which photons depart from the sub-solar 
               point's location at the light-time corrected epoch 
               et-lt and *arrive* at the observer's location at `et': 
 
                  "LT"       Correct for one-way light time (also 
                             called "planetary aberration") using a 
                             Newtonian formulation. This correction 
                             yields the location of sub-solar 
                             point at the moment it emitted photons 
                             arriving at the observer at `et'. 
  
                             The light time correction uses an 
                             iterative solution of the light time 
                             equation. The solution invoked by the 
                             "LT" option uses one iteration. 
 
                             The target position and orientation as 
                             seen by the observer are corrected for 
                             light time. The position of the Sun 
                             relative to the target is corrected for 
                             one-way light time between the Sun and 
                             target. 
 
                  "LT+S"     Correct for one-way light time and 
                             stellar aberration using a Newtonian 
                             formulation. This option modifies the 
                             sub-solar point obtained with the "LT" 
                             option to account for the observer's 
                             velocity relative to the solar system 
                             barycenter. These corrections yield 
                             the apparent sub-solar point. 
 
                  "CN"       Converged Newtonian light time 
                             correction. In solving the light time 
                             equation, the "CN" correction iterates 
                             until the solution converges. Both the 
                             position and rotation of the target 
                             body, and the position of the Sun, are 
                             corrected for light time. 
 
                  "CN+S"     Converged Newtonian light time and 
                             stellar aberration corrections. This 
                             option produces a solution that is at 
                             least as accurate at that obtainable 
                             with the "LT+S" option. Whether the 
                             "CN+S" solution is substantially more 
                             accurate depends on the geometry of the 
                             participating objects and on the 
                             accuracy of the input data. In all 
                             cases this routine will execute more 
                             slowly when a converged solution is 
                             computed. 
 
               Neither case nor white space are significant in 
               `abcorr'. For example, the string  
 
                 "Lt + s" 
 
               is valid. 
 
 
   obsrvr      is the name of the observing body. The observing body 
               is an ephemeris object: it typically is a spacecraft, 
               the earth, or a surface point on the earth. `obsrvr' is 
               case-insensitive, and leading and trailing blanks in 
               `obsrvr' are not significant. Optionally, you may 
               supply a string containing the integer ID code for 
               the object. For example both "MOON" and "301" are 
               legitimate strings that indicate the Moon is the 
               observer. 
 
               The observer may coincide with the target. 
 
-Detailed_Output
 
 
   spoint      is the sub-solar point on the target body.  
 
               For target shapes modeled by ellipsoids, the 
               sub-solar point is defined either as the point on the 
               target body that is closest to the sun, or the target 
               surface intercept of the line from the sun to the 
               target's center. 
 
               For target shapes modeled by topographic data 
               provided by DSK files, the sub-solar point is defined 
               as the target surface intercept of the line from the 
               sun to either the nearest point on the reference 
               ellipsoid, or to the target's center. If multiple 
               such intercepts exist, the one closest to the sun is 
               selected. 
 
               The input argument `method' selects the target shape 
               model and sub-solar point definition to be used. 
  
               `spoint' is expressed in Cartesian coordinates, 
               relative to the body-fixed target frame designated by 
               `fixref'. The body-fixed target frame is evaluated at 
               the sub-solar point epoch `trgepc' (see description 
               below). 
 
               When aberration corrections are used, `spoint' is 
               computed using target body position and orientation 
               that have been adjusted for the corrections 
               applicable to `spoint' itself rather than to the target 
               body's center. In particular, if the stellar 
               aberration correction applicable to `spoint' is 
               represented by a shift vector S, then the light-time 
               corrected position of the target is shifted by S 
               before the sub-solar point is computed. 
                
               The components of `spoint' have units of km. 
 
 
   trgepc      is the "sub-solar point epoch." `trgepc' is defined as 
               follows: letting `lt' be the one-way light time between 
               the observer and the sub-solar point, `trgepc' is 
               either the epoch et-lt or `et' depending on whether the 
               requested aberration correction is, respectively, for 
               received radiation or omitted. `lt' is computed using 
               the method indicated by `abcorr'. 
 
               `trgepc' is expressed as seconds past J2000 TDB. 
 
 
   srfvec      is the vector from the observer's position at `et' to 
               the aberration-corrected (or optionally, geometric) 
               position of `spoint', where the aberration corrections 
               are specified by `abcorr'. `srfvec' is expressed in the 
               target body-fixed reference frame designated by 
               `fixref', evaluated at `trgepc'. 
  
               The components of `srfvec' are given in units of km. 
 
               One can use the CSPICE function vnorm_c to obtain the 
               distance between the observer and `spoint': 
 
                  dist = vnorm_c( srfvec );
 
               The observer's position `obspos', relative to the 
               target body's center, where the center's position is 
               corrected for aberration effects as indicated by 
               `abcorr', can be computed via the call: 
 
                  vsub_c ( spoint, srfvec, obspos ); 
 
               To transform the vector `srfvec' from a reference frame 
               `fixref' at time `trgepc' to a time-dependent reference 
               frame `ref' at time `et', the routine pxfrm2_c should be 
               called. Let `xform' be the 3x3 matrix representing the 
               rotation from the reference frame `fixref' at time 
               `trgepc' to the reference frame `ref' at time `et'. Then 
               `srfvec' can be transformed to the result `refvec' as 
               follows: 
 
                   pxfrm2_c ( fixref, ref,    trgepc, et, xform );
                   mxv_c    ( xform,  srfvec, refvec );
 
 
-Parameters
 
   None. 
 
-Exceptions
 
 
   1)  If the specified aberration correction is unrecognized, the 
       error will be diagnosed and signaled by a routine in the call 
       tree of this routine. 
 
       If transmission aberration corrections are specified, the 
       error SPICE(NOTSUPPORTED) is signaled. 
 
   2)  If either the target or observer input strings cannot be 
       converted to an integer ID code, the error 
       SPICE(IDCODENOTFOUND) is signaled. 
 
   3)  If the input target body-fixed frame `fixref' is not 
       recognized, the error SPICE(NOFRAME) is signaled. A frame 
       name may fail to be recognized because a required frame 
       specification kernel has not been loaded; another cause is a 
       misspelling of the frame name. 
 
   4)  If the input frame `fixref' is not centered at the target body, 
       the error SPICE(INVALIDFRAME) is signaled. 
 
   5)  If the input argument `method' is not recognized, the error 
       SPICE(INVALIDMETHOD) is signaled by this routine, or the 
       error is diagnosed by a routine in the call tree of this 
       routine. 
 
       If the sub-solar point type is not specified or is not 
       recognized, the error SPICE(INVALIDSUBTYPE) is signaled. 
 
   6)  If insufficient ephemeris data have been loaded prior to 
       calling subslr_c, the error will be diagnosed and signaled by a 
       routine in the call tree of this routine. Note that when 
       light time correction is used, sufficient ephemeris data must 
       be available to propagate the states of observer, target, and 
       the Sun to the solar system barycenter. 
 
   7)  If the computation method specifies an ellipsoidal target 
       shape and triaxial radii of the target body have not been 
       loaded into the kernel pool prior to calling subslr_c, the 
       error will be diagnosed and signaled by a routine in the call 
       tree of this routine. 
 
   8)  The target must be an extended body, and must have a shape 
       for which a sub-solar point can be defined. 
 
       If the target body's shape is modeled as an ellipsoid, and if 
       any of the radii of the target body are non-positive, the 
       error will be diagnosed and signaled by routines in the call 
       tree of this routine. 
 
       If the target body's shape is modeled by DSK data, the shape 
       must be such that the specified sub-solar point definition is 
       applicable. For example, if the target shape is a torus, both 
       the NADIR and INTERCEPT definitions might be inapplicable, 
       depending on the relative locations of the sun and target. 
 
   9)  If PCK data specifying the target body-fixed frame 
       orientation have not been loaded prior to calling subslr_c, 
       the error will be diagnosed and signaled by a routine in the 
       call tree of this routine. 
 
   10) If `method' specifies that the target surface is represented by 
       DSK data, and no DSK files are loaded for the specified 
       target, the error is signaled by a routine in the call tree 
       of this routine. 
 
   12) If `method' specifies that the target surface is represented 
       by DSK data, and the ray from the observer to the 
       sub-observer point doesn't intersect the target body's 
       surface, the error SPICE(SUBPOINTNOTFOUND) will be signaled. 
 
   13) In some very rare cases, the surface intercept on the  
       target body's reference ellipsoid of the observer to target 
       center vector may not be computable. In these cases the 
       error SPICE(DEGENERATECASE) is signaled. 
 
   14) If the target body is the sun, the error SPICE(INVALIDTARGET) 
       is signaled. 
 
-Files
 
   Appropriate kernels must be loaded by the calling program before 
   this routine is called. 
 
   The following data are required: 
 
      - SPK data: ephemeris data for target, observer, and Sun must 
        be loaded. If aberration corrections are used, the states of 
        target, observer, and the Sun relative to the solar system 
        barycenter must be calculable from the available ephemeris 
        data. Typically ephemeris data are made available by loading 
        one or more SPK files via furnsh_c. 
 
      - Target body orientation data: these may be provided in a text or 
        binary PCK file. In some cases, target body orientation may
        be provided by one more more CK files. In either case, data
        are made available by loading the files via furnsh_c.
 
      - Shape data for the target body: 
               
          PCK data:  
 
             If the target body shape is modeled as an ellipsoid, 
             triaxial radii for the target body must be loaded into 
             the kernel pool. Typically this is done by loading a 
             text PCK file via furnsh_c. 
 
             Triaxial radii are also needed if the target shape is 
             modeled by DSK data, but the DSK NADIR method is 
             selected. 
 
          DSK data:  
 
             If the target shape is modeled by DSK data, DSK files 
             containing topographic data for the target body must be 
             loaded. If a surface list is specified, data for at 
             least one of the listed surfaces must be loaded. 
 
   The following data may be required: 
 
      - Frame data: if a frame definition is required to convert the 
        observer and target states to the body-fixed frame of the 
        target, that definition must be available in the kernel 
        pool. Typically the definition is supplied by loading a 
        frame kernel via furnsh_c. 
 
      - Surface name-ID associations: if surface names are specified 
        in `method', the association of these names with their 
        corresponding surface ID codes must be established by  
        assignments of the kernel variables 
 
           NAIF_SURFACE_NAME 
           NAIF_SURFACE_CODE 
           NAIF_SURFACE_BODY 
 
        Normally these associations are made by loading a text 
        kernel containing the necessary assignments. An example 
        of such an assignment is 
 
           NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG'
           NAIF_SURFACE_CODE += 1                     
           NAIF_SURFACE_BODY += 499                     
 
      - SCLK data: if the target body's orientation is provided by
        CK files, an associated SCLK kernel must be loaded.

   In all cases, kernel data are normally loaded once per program 
   run, NOT every time this routine is called. 
 
-Particulars
 
   There are two different popular ways to define the sub-solar 
   point: "nearest point on target to the Sun" or "target surface 
   intercept of the line containing the Sun and target." These 
   coincide when the target is spherical and generally are distinct 
   otherwise. 
 
   This routine computes light time corrections using light time 
   between the observer and the sub-solar point, as opposed to the 
   center of the target. Similarly, stellar aberration corrections 
   done by this routine are based on the direction of the vector 
   from the observer to the light-time corrected sub-solar point, 
   not to the target center. This technique avoids errors due to the 
   differential between aberration corrections across the target 
   body. Therefore it's valid to use aberration corrections with 
   this routine even when the observer is very close to the 
   sub-solar point, in particular when the observer to sub-solar 
   point distance is much less than the observer to target center 
   distance. 
    
   When comparing sub-solar point computations with results from 
   sources other than SPICE, it's essential to make sure the same 
   geometric definitions are used. 
 
 
   Using DSK data 
   ============== 
 
      DSK loading and unloading 
      ------------------------- 
 
      DSK files providing data used by this routine are loaded by 
      calling furnsh_c and can be unloaded by calling unload_c or 
      kclear_c. See the documentation of furnsh_c for limits on numbers 
      of loaded DSK files. 
 
      For run-time efficiency, it's desirable to avoid frequent 
      loading and unloading of DSK files. When there is a reason to 
      use multiple versions of data for a given target body---for 
      example, if topographic data at varying resolutions are to be 
      used---the surface list can be used to select DSK data to be 
      used for a given computation. It is not necessary to unload 
      the data that are not to be used. This recommendation presumes 
      that DSKs containing different versions of surface data for a 
      given body have different surface ID codes. 
 
 
      DSK data priority 
      ----------------- 
 
      A DSK coverage overlap occurs when two segments in loaded DSK 
      files cover part or all of the same domain---for example, a 
      given longitude-latitude rectangle---and when the time 
      intervals of the segments overlap as well. 
 
      When DSK data selection is prioritized, in case of a coverage 
      overlap, if the two competing segments are in different DSK 
      files, the segment in the DSK file loaded last takes 
      precedence. If the two segments are in the same file, the 
      segment located closer to the end of the file takes 
      precedence. 
 
      When DSK data selection is unprioritized, data from competing 
      segments are combined. For example, if two competing segments 
      both represent a surface as sets of triangular plates, the 
      union of those sets of plates is considered to represent the 
      surface.  
 
      Currently only unprioritized data selection is supported. 
      Because prioritized data selection may be the default behavior 
      in a later version of the routine, the UNPRIORITIZED keyword is 
      required in the `method' argument. 
 
       
      Syntax of the `method' input argument 
      ----------------------------------- 
 
      The keywords and surface list in the `method' argument 
      are called "clauses." The clauses may appear in any 
      order, for example 
 
         "NADIR/DSK/UNPRIORITIZED/<surface list>"
         "DSK/NADIR/<surface list>/UNPRIORITIZED"
         "UNPRIORITIZED/<surface list>/DSK/NADIR"
 
      The simplest form of the `method' argument specifying use of 
      DSK data is one that lacks a surface list, for example: 
 
         "NADIR/DSK/UNPRIORITIZED" 
         "INTERCEPT/DSK/UNPRIORITIZED" 
 
      For applications in which all loaded DSK data for the target 
      body are for a single surface, and there are no competing 
      segments, the above strings suffice. This is expected to be 
      the usual case. 
 
      When, for the specified target body, there are loaded DSK 
      files providing data for multiple surfaces for that body, the 
      surfaces to be used by this routine for a given call must be 
      specified in a surface list, unless data from all of the 
      surfaces are to be used together. 
 
      The surface list consists of the string 
 
         "SURFACES = "
 
      followed by a comma-separated list of one or more surface 
      identifiers. The identifiers may be names or integer codes in 
      string format. For example, suppose we have the surface 
      names and corresponding ID codes shown below: 
 
         Surface Name                              ID code 
         ------------                              ------- 
         "Mars MEGDR 128 PIXEL/DEG"                1 
         "Mars MEGDR 64 PIXEL/DEG"                 2 
         "Mars_MRO_HIRISE"                         3 
 
      If data for all of the above surfaces are loaded, then 
      data for surface 1 can be specified by either 
 
         "SURFACES = 1" 
 
      or 
 
         "SURFACES = \"Mars MEGDR 128 PIXEL/DEG\"" 
 
      Escaped double quotes are used to delimit the surface name
      because it contains blank characters.
          
      To use data for surfaces 2 and 3 together, any 
      of the following surface lists could be used: 
 
         "SURFACES = 2, 3" 
 
         "SURFACES = \"Mars MEGDR  64 PIXEL/DEG\", 3" 
 
         "SURFACES = 2, Mars_MRO_HIRISE" 
 
         "SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", Mars_MRO_HIRISE" 
        
      An example of a `method' argument that could be constructed 
      using one of the surface lists above is 
 
      "NADIR/DSK/UNPRIORITIZED/SURFACES= \"Mars MEGDR 64 PIXEL/DEG\",3" 
 
        
      Aberration corrections 
      ---------------------- 
 
      For irregularly shaped target bodies, the distance between the 
      observer and the nearest surface intercept need not be a 
      continuous function of time; hence the one-way light time 
      between the intercept and the observer may be discontinuous as 
      well. In such cases, the computed light time, which is found 
      using an iterative algorithm, may converge slowly or not at all. 
      In all cases, the light time computation will terminate, but 
      the result may be less accurate than expected. 
 
 
-Examples
 
 
   The numerical results shown for this example may differ across 
   platforms. The results depend on the SPICE kernels used as input, 
   the compiler and supporting libraries, and the machine specific 
   arithmetic implementation. 
 
 
   1) Find the sub-solar point on Mars as seen from the Earth for a 
      specified time.  
 
      Compute the sub-solar point using both triaxial ellipsoid 
      and topographic surface models. Topography data are provided by 
      a DSK file. For the ellipsoid model, use both the "intercept" 
      and "near point" sub-observer point definitions; for the DSK 
      case, use both the "intercept" and "nadir" definitions. 
 
      Display the locations of both the sun and the sub-solar 
      point relative to the center of Mars, in the IAU_MARS 
      body-fixed reference frame, using both planetocentric and 
      planetographic coordinates. 
 
      The topographic model is based on data from the MGS MOLA DEM 
      megr90n000cb, which has a resolution of 4 pixels/degree. A 
      triangular plate model was produced by computing a 720 x 1440 
      grid of interpolated heights from this DEM, then tessellating 
      the height grid. The plate model is stored in a type 2 segment 
      in the referenced DSK file. 
 
      Use the meta-kernel shown below to load the required SPICE 
      kernels. 
  
 
         KPL/MK 
 
         File: subslr_ex1.tm 
 
         This meta-kernel is intended to support operation of SPICE 
         example programs. The kernels shown here should not be 
         assumed to contain adequate or correct versions of data 
         required by SPICE-based user applications. 
 
         In order for an application to use this meta-kernel, the 
         kernels referenced here must be present in the user's 
         current working directory. 
 
         The names and contents of the kernels referenced 
         by this meta-kernel are as follows: 
 
            File name                        Contents 
            ---------                        -------- 
            de430.bsp                        Planetary ephemeris 
            mar097.bsp                       Mars satellite ephemeris 
            pck00010.tpc                     Planet orientation and 
                                             radii 
            naif0011.tls                     Leapseconds 
            megr90n000cb_plate.bds           Plate model based on 
                                             MEGDR DEM, resolution 
                                             4 pixels/degree. 
 
         \begindata 
 
            KERNELS_TO_LOAD = ( 'de430.bsp', 
                                'mar097.bsp', 
                                'pck00010.tpc', 
                                'naif0011.tls', 
                                'megr90n000cb_plate.bds' ) 
         \begintext 
 
 
 
     Example code begins here. 
 

        /.
        Program subslr_ex1 
        ./
        #include <stdio.h>
        #include "SpiceUsr.h"

        int main()
        {
           /.
           Local parameters
           ./
           #define META            "subslr_ex1.tm"
           #define NMETH           4

           /.
           Local variables
           ./
           static SpiceChar      * method[NMETH] =
                                   {
                                      "Intercept/ellipsoid",
                                      "Near point/ ellipsoid",
                                      "Intercept/DSK/Unprioritized",
                                      "Nadir/DSK/Unprioritized"
                                   };

           SpiceDouble             et;
           SpiceDouble             f;
           SpiceDouble             radii  [3];
           SpiceDouble             re;
           SpiceDouble             rp;
           SpiceDouble             spclat;
           SpiceDouble             spclon;
           SpiceDouble             spcrad;
           SpiceDouble             spgalt;
           SpiceDouble             spglat;
           SpiceDouble             spglon;
           SpiceDouble             spoint [3];
           SpiceDouble             srfvec [3];
           SpiceDouble             sunlt;
           SpiceDouble             sunpos [3];
           SpiceDouble             sunst  [6];
           SpiceDouble             supcln;
           SpiceDouble             supclt;
           SpiceDouble             supcrd;
           SpiceDouble             supgal;
           SpiceDouble             supgln;
           SpiceDouble             supglt;
           SpiceDouble             trgepc;

           SpiceInt                i;
           SpiceInt                n;

           /.
           Load kernel files via the meta-kernel.
           ./
           furnsh_c ( META );

           /.
           Convert the UTC request time string to seconds past
           J2000, TDB.
           ./
           str2et_c ( "2008 aug 11 00:00:00", &et );

           /.
           Look up the target body's radii. We'll use these to
           convert Cartesian to planetographic coordinates. Use
           the radii to compute the flattening coefficient of
           the reference ellipsoid.
           ./
           bodvrd_c ( "MARS", "RADII", 3, &n, radii );

           /.
           Let `re' and `rp' be, respectively, the equatorial and
           polar radii of the target.
           ./
           re = radii[0];
           rp = radii[2];

           f  = ( re - rp ) / re;

           /.
           Compute the sub-solar point using light time and stellar
           aberration corrections. Use the "target surface intercept"
           definition of the sub-solar point on the first loop
           iteration, and use the "near point" definition on the
           second.
           ./

           for ( i = 0;  i < NMETH;  i++ )
           {
              subslr_c ( method[i],
                         "mars",  et,     "iau_mars", "cn+s",
                         "earth", spoint, &trgepc,    srfvec );

              /.
              Convert the sub-observer point's rectangular coordinates
              to planetographic longitude, latitude and altitude.
              Convert radians to degrees.
              ./
              recpgr_c ( "mars",  spoint,  re,     f,
                         &spglon, &spglat, &spgalt   );

              spglon *= dpr_c();
              spglat *= dpr_c();

              /.
              Convert the sub-solar point's rectangular coordinates to
              planetocentric radius, longitude, and latitude. Convert
              radians to degrees.
              ./
              reclat_c ( spoint, &spcrad, &spclon, &spclat );

              spclon *= dpr_c();
              spclat *= dpr_c();

              /.
              Compute the Sun's apparent position relative to the 
              sub-solar point at `trgepc'. Add the position of the
              sub-solar point relative to the target's center to
              obtain the position of the sun relative to the target's
              center. Express the latter position in planetographic 
              coordinates.
              ./
              spkcpo_c ( "sun",   trgepc,  "iau_mars", "OBSERVER",
                         "lt+s",  spoint,  "mars",     "iau_mars",
                         sunst,   &sunlt                          );

              vadd_c ( sunst, spoint, sunpos );

              recpgr_c ( "mars",  sunpos,  re,     f,
                         &supgln, &supglt, &supgal    );

              supgln *= dpr_c ();
              supglt *= dpr_c ();

              /.
              Convert the Sun's rectangular coordinates to
              planetocentric radius, longitude, and latitude.
              Convert radians to degrees.
              ./
              reclat_c ( sunpos, &supcrd, &supcln, &supclt );

              supcln *= dpr_c();
              supclt *= dpr_c();

              /.
              Write the results.
              ./
              printf ( "\n"
                       " Computation method = %s\n\n"
                       "  Sub-solar point altitude            (km) = %21.9f\n"
                       "  Sub-solar planetographic longitude (deg) = %21.9f\n"
                       "  Sun's planetographic longitude     (deg) = %21.9f\n"
                       "  Sub-solar planetographic latitude  (deg) = %21.9f\n"
                       "  Sun's planetographic latitude      (deg) = %21.9f\n"
                       "  Sub-solar planetocentric longitude (deg) = %21.9f\n"
                       "  Sun's planetocentric longitude     (deg) = %21.9f\n"
                       "  Sub-solar planetocentric latitude  (deg) = %21.9f\n"
                       "  Sun's planetocentric latitude      (deg) = %21.9f\n"
                       "\n",
                       method[i], 
                       spgalt, 
                       spglon,
                       supgln, 
                       spglat, 
                       supglt, 
                       spclon, 
                       supcln,
                       spclat,
                       supclt      );
           }
           return ( 0 );
        }

 
   When this program was executed on a PC/Linux/gcc 64-bit 
   platform, the output was: 
 

      Computation method = Intercept/ellipsoid

       Sub-solar point altitude            (km) =           0.000000000
       Sub-solar planetographic longitude (deg) =         175.810675508
       Sun's planetographic longitude     (deg) =         175.810675508
       Sub-solar planetographic latitude  (deg) =          23.668550281
       Sun's planetographic latitude      (deg) =          23.420823362
       Sub-solar planetocentric longitude (deg) =        -175.810675508
       Sun's planetocentric longitude     (deg) =        -175.810675508
       Sub-solar planetocentric latitude  (deg) =          23.420819936
       Sun's planetocentric latitude      (deg) =          23.420819936


      Computation method = Near point/ ellipsoid

       Sub-solar point altitude            (km) =           0.000000000
       Sub-solar planetographic longitude (deg) =         175.810675408
       Sun's planetographic longitude     (deg) =         175.810675408
       Sub-solar planetographic latitude  (deg) =          23.420823362
       Sun's planetographic latitude      (deg) =          23.420823362
       Sub-solar planetocentric longitude (deg) =        -175.810675408
       Sun's planetocentric longitude     (deg) =        -175.810675408
       Sub-solar planetocentric latitude  (deg) =          23.175085578
       Sun's planetocentric latitude      (deg) =          23.420819936


      Computation method = Intercept/DSK/Unprioritized

       Sub-solar point altitude            (km) =          -4.052254284
       Sub-solar planetographic longitude (deg) =         175.810675512
       Sun's planetographic longitude     (deg) =         175.810675512
       Sub-solar planetographic latitude  (deg) =          23.668848891
       Sun's planetographic latitude      (deg) =          23.420823362
       Sub-solar planetocentric longitude (deg) =        -175.810675512
       Sun's planetocentric longitude     (deg) =        -175.810675512
       Sub-solar planetocentric latitude  (deg) =          23.420819936
       Sun's planetocentric latitude      (deg) =          23.420819936


      Computation method = Nadir/DSK/Unprioritized

       Sub-solar point altitude            (km) =          -4.022302438
       Sub-solar planetographic longitude (deg) =         175.810675412
       Sun's planetographic longitude     (deg) =         175.810675412
       Sub-solar planetographic latitude  (deg) =          23.420823362
       Sun's planetographic latitude      (deg) =          23.420823362
       Sub-solar planetocentric longitude (deg) =        -175.810675412
       Sun's planetocentric longitude     (deg) =        -175.810675412
       Sub-solar planetocentric latitude  (deg) =          23.174793924
       Sun's planetocentric latitude      (deg) =          23.420819936
 
-Restrictions
 
  None. 
 
-Literature_References
 
   None. 
 
-Author_and_Institution
 
   N.J. Bachman   (JPL) 
   S.C. Krening   (JPL) 
   B.V. Semenov   (JPL) 
 
-Version
 
   -CSPICE Version 3.0.0, 05-APR-2017 (NJB) (SCK) (BVS)

       Fixed some header comment typos.

    16-OCT-2015

       Updated to support use of DSKs.

   -CSPICE Version 1.1.0, 08-MAY-2012 (NJB) (SCK)
 
       Some changes to computation of the surface point-to-sun vector
       were made in the underlying SPICELIB routine SUBSLR. These
       changes have a small effect on the outputs of this routine.
  
       Exceptions removed: the observer and target are now
       permitted to coincide.

       The header example program was updated to reflect the new
       method of computing the apparent sun location, and the set
       of kernels referenced by the example meta-kernel were updated.
       The display of the program's output was updated accordingly.

       References to the new pxfrm2_c routine were added to the
       Detailed Output section.

   -CSPICE Version 1.0.1, 06-FEB-2009 (NJB)
 
       Incorrect frame name fixfrm was changed to fixref in
       documentation.
 
       In the header examples, meta-kernel names were updated to use
       the suffix

          ".tm"
 
   -CSPICE Version 1.0.0, 02-MAR-2008 (NJB) 

-Index_Entries
 
   find sub-solar point on target body 
   find nearest point to sun on target body 
 
-&
*/

{ /* Begin subslr_c */


   /*
   Participate in error tracing.
   */
   chkin_c ( "subslr_c" );

   /*
   Check the input strings: method, target, fixref, abcorr, and obsrvr.
   Make sure none of the pointers are null and that each string
   contains at least one non-null character.
   */
   CHKFSTR ( CHK_STANDARD, "subslr_c", method );
   CHKFSTR ( CHK_STANDARD, "subslr_c", target );
   CHKFSTR ( CHK_STANDARD, "subslr_c", fixref );
   CHKFSTR ( CHK_STANDARD, "subslr_c", abcorr );
   CHKFSTR ( CHK_STANDARD, "subslr_c", obsrvr );
 
   /*
   Call the f2c'd routine.
   */
   subslr_ ( ( char         * ) method,
             ( char         * ) target,
             ( doublereal   * ) &et,
             ( char         * ) fixref,
             ( char         * ) abcorr,
             ( char         * ) obsrvr,
             ( doublereal   * ) spoint,
             ( doublereal   * ) trgepc,
             ( doublereal   * ) srfvec,
             ( ftnlen         ) strlen(method),
             ( ftnlen         ) strlen(target),
             ( ftnlen         ) strlen(fixref),
             ( ftnlen         ) strlen(abcorr),
             ( ftnlen         ) strlen(obsrvr)  );

   chkout_c ( "subslr_c" );

} /* End subslr_c */
